Recently, a novel step-scheme (S-scheme) heterojunction was proposed and has attracted researchers' attention. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] Usually, S-scheme heterojunction consists of reduction photocatalyst (RP) and oxidation photocatalyst (OP). Besides, the directional migration of free electrons will lead to band bending and internal electric field (IEF) at their interface owing to the work function difference. Notably, under the influence of IEF, the photogenerated electrons of OP with weak reduction ability can recombine with the photogenerated holes of RP with weak oxidation ability; while those with strong redox abilities are preserved. Therefore, reasonable construction of TiO 2 -based S-scheme heterojunction is of great significance to improve photocatalytic reaction performance. Apart from the charge carrier separation, the morphology of photocatalysts is another important factor to influence the photocatalytic performance. [15,17,19,37,38] Photocatalysts with hollow structures have attracted great attention owing to manifold advantages including larger specific surface area, abundant active sites, shortened diffusion distance as well as improved light reflection and scattering. [37,[39][40][41][42][43][44] Therefore, the design of hollow S-scheme heterojunction photocatalyst is of vital importance to enhance photocatalytic performance.ZnIn 2 S 4 , as a typical reduction photocatalyst, stands out for its layered structure, narrow bandgap, suitable redox potentials, and good chemical stability. And it has been used for various photocatalytic applications including hydrogen production, CO 2 reduction, and organic degradation. [45][46][47][48][49][50][51] Unfortunately, pristine ZnIn 2 S 4 photocatalyst shows low photocatalytic efficiency owing to the fast recombination of photogenerated charge carriers. [45][46][47][48][49] Considering the suitable match of band gap of ZnIn 2 S 4 and TiO 2 for S-scheme heterojunction, [27,52] we construct the hollow TiO 2 @ZnIn 2 S 4 core-shell structure. Up to now, to the best of our knowledge, it has never been reported.Herein, we grow ZnIn 2 S 4 nanosheets on the outer surface of TiO 2 hollow spheres by in situ chemical bath deposition reaction. This rational design is not only able to provide large specific surface areas and abundant reaction sites for PCR reaction, but also can effectively suppress the recombination of useful photogenerated electrons and holes. As a result, the optimized TiO 2 @ZnIn 2 S 4 heterojunction exhibits high PCR performance, and the total CO 2 photoreduction conversion rates (the sum yield of CO, CH 3 OH and CH 4 ) are obviously higher than those of blank ZnIn 2 S 4 , TiO 2 , and ex situ prepared TiO 2 -ZnIn 2 S 4 composite. Finally, S-scheme mechanism is also thoroughly analyzed and discussed in this work.Reasonable design of efficient hierarchical photocatalysts has gained significant attention. Herein, a step-scheme (S-scheme) core-shell TiO 2 @ZnIn 2 S 4 heterojunction is designed for photocatalytic CO 2 redu...

A novel high-purity V 2 C MXene two-dimensional carbide, was successfully synthesized by etching V 2 AlC with sodium fluoride and hydrochloric acid at 90 • C for 72 h. From the analysis of X-ray diffraction, energy dispersive spectra, and X-ray photoelectron spectroscopy, the purity of as-synthesized V 2 C MXene was >90 wt% with a few impurities of Na 5 Al 3 F 14 and V 2 AlC. The V 2 C MXene made by this method was much purer than those made by HF etching at room temperature. The as-prepared V 2 C MXene showed excellent electrochemical properties as anode of lithium-ion batteries. The capacity can be 260 mAh g −1 if discharged under 370 mA g −1 . The capacity was increased with charge cycles at high charge rate (500 mA g −1 ). It was suggested that V 2 C with high purity can be promising anode material with excellent performance.

The effect of etching solution on the synthesis process of two-dimensional vanadium carbide (V2C MXene) was researched. Three etching solutions were used to etch ternary carbide V2AlC at 90 °C. The three solutions were: lithium fluoride + hydrochloric acid (LiF + HCl), sodium fluoride + hydrochloric acid (LiF + HCl), and potassium fluoride + hydrochloric acid (KF + HCl). It was found that only NaF + HCl solution was effective for synthesizing highly pure V2C MXene. The existence of sodium (Na+) and chloridion (Cl−) in etching solution was essential for the synthesis. The thermal stability of the as-prepared V2C MXene in argon or air was studied. From thermogravimetry and differential thermal analysis, V2C MXene was found to be stable in argon atmosphere at a temperature of up to 375 °C. As the temperature increased, V2C MXene was gradually oxidized to form nanoparticles composed of vanadium trioxide (V2O3) and a part of V2C MXene was broken and transformed to vanadium carbide (V8C7) at 1000 °C. In air atmosphere, V2C MXene was stable at 150 °C. At 1000 °C, V2C MXene was oxidized to form vanadium pentoxide (V2O5).

The development of 2-dimensional materials has expanded beyond the realm of graphene, and now includes inorganic 2-dimensional transition metal oxides/hydroxides, which show promise for a wide range of applications. As an emerging class of nanoscale materials, they show unprecedented properties that are unattainable in their bulk lamellar systems, which can be attributed to their confined thickness compared to several tens of micrometer lateral dimensions. Such qualities make them viable candidates for battery and supercapacitor applications. There are a few challenges ahead for 2-dimensional transition metal oxides/hydroxides, including the limited types of 2-dimensional parent materials in bulk form, the controlled synthesis of 2-dimensional nanostructures with nonlayered structures, and the ability to control the properties of layers by tuning the chemistry and nanoscopic features. This Focus Review will cover the research landscape of 2-dimensional transition metal oxides/ hydroxides, ranging from synthetic approaches, to understanding the properties that emerge at the single-layer scale, to exploiting these properties in both new and existing technologies.

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